In the last decades, progress in radio and VLSI (Very Large Scale Integration) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as portable radios, mobile radio communication units, and the like can now be produced having acceptable costs, size and power consumption.
Mobile telephone communications for the consumer market started with cellular telephone systems derived from the police and rescue services, based on an analog technology improved and optimized in the seventies and eighties. Examples of these early analog mobile telephone systems are indicated by the acronyms NMT and TACS.
Typically, cellular communication systems comprise a plurality of mobile or portable radio communication units and a plurality of radio access units or base stations. Each access unit provides a number of radio communication channels to a geographically limited area, called a communication cell, defined by the operating ranges of a particular radio access unit. The radio access units connect to a central interface unit, also called Radio Exchange (RE) or Mobile Telephone Switching Office (MTSO). The RE or MTSO, in turn, connects to a Public Switched Telephone Network (PSTN) or Integrated Services Digital Network (ISDN) for completing calls between mobile radio users and landline subscribers. A call of a mobile radio user is handed over from cell to cell while the user moves in the coverage area of the radio communication system.
The usage of cellular mobile telephones really took off in the nineties, by the introduction of mobile telephone systems based on digital technology, known under the acronyms GSM, D-AMPS (US-TDMA or IS-136), IS-95 and PDC. Extensions to GSM (also to be applied in D-AMPS) like EDGE have been introduced to improve the data rate on the digital air interfaces.
Generally, the analog systems are indicated as being the first generation, whereas the digital systems are indicated as second generation. Recently, third generation cellular systems have been developed, indicated by the acronyms UMTS, IMT-2000, CDMA2000, featuring larger transmission bandwidths.
While deploying the different systems, extensive work has been carried out to increase the capacity of the systems. Since licensed RF spectrum is in use, the more users the operator can accommodate per MHz-bandwidth, the more efficient he uses his precious and scarce resources, thereby improving his revenues.
A known method of increasing transmission capacity is by applying directional antennas. Early radio communication systems already applied the technique of sectorization, wherein a communication cell is divided into segments of 60 or 120 degrees, each segment covered by a directional antenna beam. In principle, this relates to cell splitting with the nice advantage that no additional radio access sites have to be built: one omni-directional antenna of a communication cell is replaced by six or three directional antennas mounted on the same site installation.
More advanced systems make use of radio access units having adaptive or smart antennas, which can steer the antenna beam towards the mobile user. As the mobile radio user moves, a tracking mechanism is applied in order to keep the beam pointing towards the mobile user. In addition to pointing the beam to the desired user, nulls can be steered towards interfering users such that their signals are greatly suppressed by the antenna diagram. However, when the angle of arrival between the intended and interfering signal is rather small, no optimal setting can be found.
In other cellular communication systems, extensive use is made of adaptive antennas. The capacity of a cellular telephone system is greatly improved by applying Space Division Multiple Access (SDMA). In SDMA, the communication channel is formed by a fixed geographical area: only from this area, signals can arrive.
In advanced cellular systems, adaptive antennas are used the antenna patterns of which can be updated dynamically to define the area covered. In this case, the geographical area is not fixed, but depends on the mobile user position relative to the base station position.
Other mechanisms applied to increase the capacity comprise joint demodulation. By demodulating interferers and taking them into account while demodulating the desired signal, the demodulation can be greatly improved. Because the system can then operate under lower SNR conditions, the capacity of the system increases.